In recent years, in processes for manufacturing semiconductors, liquid crystal devices, and the like, rapid progress has been made in formation of a finer pattern using lithography. Examples of technology for formation of a finer pattern include a technology using shorter wave radiation.
In recent years, KrF excimer laser (wavelength: 248 nm) lithographic technology has been introduced. Also, ArF excimer laser (wavelength: 193 nm) lithographic technology and EUV (wavelength: 13.5 nm) lithographic technology, which are intended to use shorter wavelengths, have been investigated.
Furthermore, for example, a so-called chemical amplification type resist has been proposed as a resist compound suitably applicable to shorten the wavelength of irradiation light and to pattern microfabrication. Such a chemical amplification type resist includes a polymer, which becomes soluble in alkali when an acid-eliminable group is dissociated by the action of an acid, and a photoacid generator. The resist composition has been further developed and improved.
An acrylic type polymer transparent to light with a wavelength of 193 nm has attracted attention as a chemical amplification resist polymer used in ArF excimer laser lithography.
For example, copolymers for lithography as described in Patent Document 1 below are produced using, as monomers, (A) a (meth)acrylate to which an aliphatic hydrocarbon having a lactone ring is ester-bonded, (B) a (meth)acrylate to which a group dissociable by the action of an acid is ester-bonded, and (C) a (meth)acrylate to which a hydrocarbon group or an oxygen atom-containing heterocyclic group having a polar substituent is ester-bonded.
In general, furthermore, a (meth)acrylate polymer is obtained by radical polymerization. Generally, in a multi-component polymer made of two or more monomers, the monomers differ in copolymerization reaction ratio. Thus, the copolymer composition ratio of the polymers in the initial stage is different from that in the last stage. Namely, the obtained polymer resultantly has a composition distribution.
When a polymer has variations in the composition ratio of constitutional units, the solubility of the copolymer tends to be less in a solvent. Thus, the preparation of a resist composition may be affected. For example, preparation of a resist composition takes a long time to dissolve the copolymer in a solvent, and causes an increase in the number of production steps due to generation of an insoluble substance. Also, the obtained resist composition tends to have insufficient sensitivity.
On the other hand, for example, a method for obtaining a polymer having a narrow copolymer composition distribution as described in Patent Document 2 below makes a difference between the feed rate of a monomer having a relatively higher polymerization rate to a monomer having a lower polymerization rate in the front end of the process and that in the back end of the process to obtain a resist having high resolution.
Also, a trace amount of a macromolecular component (high polymer) generated in the polymerization process may cause a decrease in the solubility of a polymer for lithography in a resist solvent as well as in an alkali developing solution. As a result, the sensitivity of a resist composition is decreased.
In Patent Document 3 below, a method of limiting the generation of such a polymer is proposed. In this method, a solution containing a polymerizable monomer and a solution containing a polymerization initiator are respectively held in separate reservoirs. Then, the polymerization initiator is fed earlier than the polymerizable monomer to a polymerization system.
On the other hand, attention has been focused on an acrylic type polymer transparent to light having a wavelength of 193 nm as a chemical amplification-type resist to be used in ArF excimer laser lithography. For example, a copolymer of a (meth)acrylate having an adamantine skeleton in the ester part and a (meth)acrylate having a lactone skeleton in the ester part as the above acrylic type polymer (for example, Patent Documents 4 and 5).
Incidentally, a (meth)acrylate polymer is obtained by radical polymerization. In a multi-component polymer produced from two or more types of monomers, the monomers have their respective copolymerization reaction rates. Thus, the copolymer composition ratio of the polymer in the initial stage is different from that in the last polymerization stage. Namely, the resulting polymer has a composition distribution. A copolymer having such a composition distribution tends to deteriorate resist performance. Therefore, studies have been made to control the composition distribution of a copolymer.
For example, from the viewpoint of solubility in solvent, Patent Document 6 describes that the content (mol %) of a constitutional unit derived from a (meth)acrylate monomer having a lactone skeleton in each copolymer contained in 10 to several tens of fractions obtained by dividing a copolymer solution by gel permeation chromatography (hereinafter referred to as “GPC”) is preferably within −10 to +10 mol % of the average content of a constitutional unit derived from a (meth)acrylate monomer having a lactone skeleton in the whole copolymers.
Also, from the viewpoint of the formation of a finer pattern in semiconductor lithography, Patent Document 7 describes that the molar composition of a constitutional unit having a hydroxyl group in a low-molecular-weight region corresponding to 5% of the peak of all copolymer in GPC is preferably within ±10% of the average molar composition of a constitutional unit having a hydroxyl group in all copolymer.